Leak detection and surface flaw non-destructive testing techniques often use fluorescent dye additives or fluorescent penetrants. These techniques rely upon the unique physical property of various materials to fluoresce when excited by certain wavelengths of light.
Fluorescence is generally understood to be a property that enables some materials to absorb light energy and to radiate visible light at a longer wavelength than the absorbed light. According to generally accepted theory, electrons in fluorescent materials are excited upon being illuminated by light energy of a specific absorption band wavelength, and emit light energy in a longer wavelength response band as the electrons return to the unexcited or ground state. The specific excitation and response wavelengths are characteristics of the particular fluorescent materials.
The apparent brightness of a fluorescent material's luminescence is dependent on the wavelength and the intensity of the incident radiation. The excitation band generally has one or more peak wavelengths that will produce a greater response than incident light of the same intensity at an off-peak wavelength. Thus, a fluorescent material which has an excitation peak at a specific wavelength may exhibit a much reduced luminescence as the wavelength of incident light deviates from the excitation peak, and will lose the ability to fluoresce when the incident light does not provide enough energy within the excitation range.
For example, two commonly used fluorescent leak detection dyes are perylene-based fluorescent compounds and naphthalimide-based fluorescent compounds. Perylene dyes produce a yellow fluorescent response when exposed to incident radiation which includes the UV-A wavelength band of about 315 nm to about 400 nm, with a strong peak between about 340 to 375 nm. Naphthalimide dyes fluoresce green when exposed to incident radiation of visible violet/blue light in a range from about 400 nm to about 480 nm.
A fluorescent response is more visible when the intensity of other visible light is reduced, so that the fluorescent response is not masked or washed-out by other light. The various UV-A or Violet/Blue inspection lamps use several types of light sources and filtering to produce a light output in the excitation bands with little or no output light in the fluorescent response band. For example, a lamp having a high intensity incandescent light source with a narrow band UV (360-370 nm) absorption filter will emit light energy concentrated around the excitation peak of a perylene dye additive. An inspection lamp with a wider band UV/BLUE absorption filter centered at about 400 nm provides output in the UVA and visible violet/blue range, with the greatest intensity centered in the excitation band of a common naphthalimide dye compound. In the absorption filter lamps, however, the larger portion of light energy in the visible and infrared wavelengths is absorbed as heat in the filter.
A more efficient inspection lamp uses thin-film dichroic reflectance filter. A dichroic filter can be tailored to reflect back into the lamp only the range of visible wavelengths outside of the chosen excitation band, while passing the other wavelengths. Because the emitted light is not converted to heat in the dichroic filter, the lamp can be made considerably more compact than the lamps with absorption filters.
All of the above-described prior lamps use a broad spectrum light source, and thus require some type of filtering, whether absorption or dichroic, to transmit light in an excitation band while restricting light output in the visible fluorescent response band. In the present invention, however, the inspection lamp uses the narrow band electroluminescence of solid state lamps, specifically light-emitting diodes (hereinafter referred to as “LEDs”) with glass envelopes that refract light to the tip of the envelope. LEDs have been known for many years, but until recent developments it has been difficult to obtain sufficiently high levels of luminous flux as would be required for an inspection lamp. This problem was particularly acute for LEDs emitting in the blue to UV bands, which produced much less lumens per watt than the red, yellow and green emitting LEDs. [See, Lighting Handbook, 8th edition, Illuminating Engineers Society of North America, Chapter 6, figure 6.68(f)].
Recent developments in nitride semiconductor materials, particularly gallium nitride (GaN) based epitaxal structures, have provided more efficient LEDs that can produce sufficient lumens for an inspection lamp in the UV 360-390 nm range. For example, a GaN electroluminescent device as described in U.S. Pat. No. 5,898,185 has an emission peak at 380 nm. An LED emitting in the 360 nm-390 nm range would make a useful light source for detection of a perylene dye.
GaN alloys can produce other useful emission bands. The same patent describes a commercial GaInAlN LED that emits blue light at 460 nm. The light emission wavelengths of GaN LEDs can also be altered by phosphor films if desired. As described in the specification of the U.S. Pat. No. 5,898,185, these GaN-based LEDs have high efficiencies, typically in the energy emitted/power-in range of 10%.
The invention uses these LED light sources in the form of a LED bulb in which the glass lens capsule directs light to the rounded tip of the capsule. This causes a narrow high intensity focus at the tip and a diverging beam emanating from the tip. The effect is having a highest light intensity at the capsule tip and a rapidly decreasing intensity as distance from the tip increases. Thus, while the intensity at close distances may be sufficient to excite a strong response from a fluorescent dye, the intensity at longer distances may not be enough to produce the fluorescent response.
An LED inspection lamp can be very small, in fact, it can be the size of a pen light powered by AAAA sized batteries. A current LED pen light, the STYLUS™ model with an “Ice Blue” emitting LED is only 0.38 inches in diameter and 6.60 inches in length and operates on three AAAA batteries with a run time of over 10 hours, yet it has sufficient blue light output to cause a fluorescent response in a naphthalimide-based leak detection additive from a distance of 18 inches in dark conditions. Even in daylight conditions, it can excite a naphthalimide dye to a bright response at one to six inches.
Although the narrow shape of the LED pen light is helpful in allowing the LED capsule to be inserted into narrow areas close to a suspected leak site, it is still restricted by its length. Furthermore, the metal handle and battery housing is inflexible. Both the short length and inflexibility can make it difficult or impossible to reach areas of potential leak sites, such as lines and connectors in the circuit of an automobile air conditioner.
It would therefore be useful to provide an LED lamp that has a high intensity focus at the tip as well as an extendible handle so that the lamp may be inserted deeply into tight areas to bring the tip of the LED capsule close to the potential leak site to be investigated for fluorescent material. An extendible handle may alternatively be flexible to aid in insertion around obstructions. An alternative embodiment lamp may have two or more different color LEDs, such as violet and blue, which may be selectively used to detect more than one type of fluorescent material, or may have an LED and a white light bulb to select between general illumination and fluorescent detection. An LED lamp with an extendible handle may also have a mirror attached to permit inspect under or around an obstruction.